I constructed the fully lined bottoms with the stitch-and-flip method of swimsuit construction. I prefer a fully lined suit but I like the seams clean-finished, especially in the crotch area (I never put a crotch lining in a swimsuit). Here's how to make a bikini bottom that is completely lined and no seams exposed, otherwise known as the Stitch-and-Flip Method.
First, cut out the front and back pieces of main fabric and the swimsuit lining.
Stack up the main fabric with right sides together, and the lining with right sides together. Then stack the two main fabrics on top of the lining layers. Pin all the layers together.
Serge the centre back seam together (all 4 layers). If you do not have a serger, you can use a small zig-zag stitch. This is one reason why the stitch-and-flip method is so great. Even without a serger, the seams can be neatly finished on the inside.
This is what that seam looks like outside and inside. The back is finished.
Now for the front. Pin the right side of the front fabric to the right side of the back at the crotch seam and the side seams.
Now pin the right side of the lining to the right side of the lining, again at the side seams and the crotch seam.
Serge or zig-zag those same seams.
You are done. Flip the back lining over the seams so that the lining is correctly positioned against the main fabric. Here are the seams from the inside. Everything is nice and neat.
The bottoms are ready to finish with the leg and hip elastic!